Electronic Toll Collection System Computer Science Essay

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ETC (Electronic Toll Collection) is an electronic automation toll collection system that was been a hot topic of research in recent years. RFID enabled labels are pasted on the vehicles. The center control computer can identify the road user by the information stored in the electronic label and deduct the toll from the road user advance stored card or bank account.

The RFID and ETC specifications have been issued, and the issue is being discussed all over the world. Several frequency bands are used in ETC, such as 900MHz, 2.4GHz and 5.8GHz as well. Pakistan has now joined the list of growing countries where RFID based electronic toll collection is in use. For now it has been introduced by NHA (National Highway Authority) in collaboration with NADRA (National Database and Registration Authority) on a few inter-city toll collection points and Peshawar - Islamabad M1 and Islamabad - Lahore M2 Motorways. The system which is installed in Pakistan, a vehicle will still have to stop at a booth but no human transaction between the vehicle occupants and toll booth operator is needed.

1.2 Project Objective

Antenna is an important part of the Reader and the Tag, and it exchanges message and energy between Reader and Tag. The purpose of project is to design a Microstrip patch antenna for RFID toll-road application working in ISM band, 2.4-2.5 GHz. A single patch antenna of compact size is designed for the transmitter along with the array antenna for the reader. According to the requirement of the far distance RFID ETC system, several key parameters are considered as follows:

Sufficient bandwidth to get enough data transferring velocity.

Sufficient gain to obtain read-and-write distance between the Reader and the Tag.

Good radiation pattern or directional function, the antenna should have proper main lobe width and low side-lobe level.

Circular polarization, receiving efficiency of circular polarization antenna is little influenced by the relative position of the Reader and Tag.

Appropriate size, particularly, the Tag's antenna size should be very small.

1.3 Report Outline

Chapter 2 presents the background theory specific to the subject. The chapter starts with an introduction to antennas, including its properties and applications. Various parameters related to antenna performance including gain, bandwidth, radiation pattern and polarization are also discussed. RFID basics and the electronic toll collection system are discussed in the proceed-ing sections of the chapter.

Chapter 3 is about design and simulation details of the proposed antenna.

Chapter 4 discusses the results of our antennas. It further clarifies each design specification requirement as discussed in section 1.2.

Chapter 5 concludes all the work done throughout this design project. There have been further studies and ideas which could help to explore the vastness of the subject.

Chapter 2

2 Background

2.1 Antenna Theory

All the wireless communication systems being used today contain a common element, so called antenna. An antenna is a transition between a guiding device (transmission line, waveguide) and free space (or usually another unbounded medium). Its main purpose is to convert energy of a guided wave into the energy of a free-space wave (or vice versa) as efficiently as possible, while in the same time the radiated power has a certain desired pattern of distribution in space. For an efficient antenna design certain characteristic parameters such as high bandwidth, good radiation pattern, compact dimensions, and low cost are always preferred.

Microstrip patch antennas were introduced in early 1950's [1]. These are one of the most popular planar antennas because of the ease of fabrication and integration with other devices. Microstrip antennas come in many different shapes and sizes depending upon the application and the resonant frequency used. Its applications include GPS receiver antennas, cellular phones, airplane antennas, missiles and satellite communications [1]-[2].

2.2 Fundamental Parameters of Antennas

2.2.1 Radiation Pattern

Radiation pattern of an antenna is defined as "the graphical representation of antenna power or relative field strength radiated or received by the antenna" [1]. It is the most important characteristic parameter of an antenna. An antenna's efficiency is also determined by its radiation pattern. The radiation pattern is usually calculated in the far-field region. It is represented in 3-D or 2-D. An example radiation pattern is shown in figure below:

Figure-2.1: An example radiation pattern in 3-D and 2-D

2.2.2 Beamwidth

The angular separation between two points in the radiation pattern is called beamwidth [1]. Half power bandwidth (HPBW) is defined as the angular separation between the half power points on the radiation pattern. At this point the gain of the antenna is half of the maximum value and the radiation field strength has 70.7% (3dB below) of its maximum value. Another beam width is first null beam width defined as the angular separation between the first nulls of the radiation pattern [1].

Figure-2.2: An illustration of half-power beamwidth

2.2.3 Directivity

Directivity is defined as the ratio of radiation intensity from the antenna in one direction to the average radiation intensity in all directions [2]. It can also be defined as the maximum amount of radiation in a certain direction. Mathematically, it can be represented as:

D = U/Uo = 4Ï€.U/Pt

(2.1)

where D is the directivity of antenna, U is the radiation intensity of antenna in a reference direction, Uo is the radiation intensity in all directions and Pt is the total radiated power.

When the direction of radiation is not defined, the above relationship turns out to be:

D = Umax/Uo

(2.2)

2.2.4 Gain

The directivity parameter only shows the directional properties of the antenna, whereas the gain is the measure of both the directional properties and antenna efficiency [1]-[2]. It can be defined as "the ratio of radiation intensity in a certain direction to the input power" [1]. Mathematically:

Gain = G = 4π .U(θ,φ)/Pin

(2.3)

or

G = Pr /Pt

(2.4)

2.2.5 Beam Efficiency

The total beam area is the sum of main lobe and the minor lobe [5]

(2.5)

The beam efficiency of the main lobe is defined as the ratio of beam area of the main lobe to the total beam area of an antenna [5].

(2.6)

The beam efficiency of the minor lobe is defined as the ratio of minor lobe area to the beam area of an antenna. It is also known as stray factor [5].

(2.7)

2.2.6 Bandwidth

Bandwidth is defined as the set of frequencies at which an antenna can be used effectively. This set of frequencies can be on either side of the central (resonant) frequency. Outside of this band of frequencies, the level of reactance rises to such a level that the antenna's performance is undesirable. For ETC application, the bandwidth of an antenna is of great importance as the read and write distance between the transmitter and receiver depends on it [4]. It is desirable to have antennas with greater bandwidth. There are several techniques of increasing antenna bandwidth to desirable levels. One such technique used in Microstrip patch antennas to increase their bandwidth is to thicken the substrate.

2.2.7 Polarization

Polarization is the property of EM waves that describes its orientation in free space [3]. The polarization characterizes a time varying EM wave moving in a certain direction and the magnitudes of E-field vectors. It is classified into three different types: (1) linear polarization, (2) circular polarization, and (3) elliptical polarization.

(1) Linear Polarization

If the E-field vector is moving along a line [2], or if the E-field vector is oriented on one axis only, then the polarization said to be linear polarization. The following figure illustrates linear polarization.

Linear polarization has further two types: vertical polarization and horizontal polarization.

Vertical Polarization: A wave is said to be vertically polarized if its electric field vector is directed perpendicular to its surface [3].

Horizontal Polarization: A wave is said to be horizontally polarized if its electric field vector is directed horizontally to a surface [3].

Figure-2.3: Linear Polarization

(2) Circular Polarization

Circular polarization takes place when the magnitudes of both components are same i.e. E1 = E2 [1]. In this case the wave is travelling in both the vertical and horizontal direction with a phase difference of 90o between them. Research has shown that antennas used in the user mobile equipments are essentially circularly polarized. The concept of circular polarization is illustrated in Figure-2.4.

Figure-2.4: Circular Polarization

(3) Elliptical Polarization

When both the field components are of different magnitudes and having a phase difference of 90o between them, then the wave is said to be elliptically polarized wave [1]. Circular polari-zation is a special case of elliptical polarization.

2.2.8 Input Impedance

It is defined as "the impedance present at the terminals of an antenna or the ratio of electric and magnetic field components at some point" [1]. Normally the input impedance of the transmission line through which the antenna is feed is 50Ω; however, the input impedance of antenna itself can vary. This can cause the signal to reflect back, hence reducing the efficiency of the antenna.

2.2.9 Antenna Efficiency

Antenna efficiency is calculated due to the losses occurring at the input terminals of the antenna and within the rest of the antenna structure as well. These losses occur due to reflecting back of signal and due to dielectric and conduction losses. Mathematically it can be represented as [1]:

eo = er .ec .ed

(2.8)

where eo is the total antenna efficiency, er is the reflection efficiency, ec is the conduction efficiency, and ed is the dielectric efficiency.

2.3 Microstrip Patch Antenna

2.3.1 General Theory and Design Equations

Microstrip patch antennas are widely popular due to their low profile, inexpensive while manufacturing, straightforward and comfortable in performance on both planer and non-planer surfaces. They are very flexible in terms of polarization, radiation patterns and impedances. They are generally known to have good bandwidths and are used in high-tech applications like satellite communication, aircrafts, etc. [1]-[5]. They usually consist of rectangular patch or other forms like circular or square, on a layer of substrate (dielectric), which itself is on a ground plane as shown in figure below [5].

Figure-2.4: A Microstrip Patch Antenna

The major characteristic parameters of a microstrip patch antenna are the length (L), width (W) of its rectangular patch and the height (h) of the substrate. An antenna desired for good performance should have lower dielectric constant (εr) and thicker substrate. In this way, better bandwidths and improved efficiency are achieved [5].

Microstrip patch antennas consist of a dielectric substrate sandwiched between a radiating patch and a ground plane. The radiating patch can be of different sizes and shapes depending on the operating frequency and application, while the size and shape of the ground plane is same as that of the substrate. The patch is designed in such a way that the radiations are maximum normal to the patch. The length of a rectangular patch is usually λ/3 < L < λ/ 2 [5]. The resonant frequency often decides the actual size of an antenna. For an antenna operating at higher frequencies, a smaller patch size and a thick substrate are required.

The formulas used to calculate length and width of a rectangular microstrip antenna are:

For width:

(2.9)

For length:

(2.10)

where ΔL is the extension in length of the patch due to fringe field, given as:

(2.11)

where the effective dielectric constant εeff is given by:

(2.12)

2.3.2 Feeding Techniques

There are different ways to feed a patch antenna. The most common of them are microstrip line feed, coaxial probe feed, aperture coupled feed, and proximity coupled feed.

(1) Microstrip line feed

Transmission line feed or microstrip line feed consists of a strip placed at the edge of the radiating patch. The width of the strip is smaller as compared to the radiating patch. This technique is the simplest of all feeding techniques used in microstrip antennas and is the easiest to fabricate [1]. If the length of the strip exceeds the wavelength, losses will occur. To reduce these losses, the strip should have a substrate with a high dielectric constant and low height, so that the fields are restricted to the strip line [5]. The strip length and width should be kept such that the input impedance is maintained at 50Ω. A microstrip line fed antenna is shown in figure below:

Microstrip line feed

Substrate

Figure-2.5: Microstrip line feed

(2) Coaxial probe feed

It consists of inner and outer conductors. The inner conductor is connected to the radiating patch while the outer conductor is attached to the ground plane. The coaxial probe is easy to fabricate but like micro strip line feed; it has narrow bandwidth and is hard to model for thick substrates. Matching becomes difficult for thicker substrate because of increased length of probe, making it more inductive. To reduce inductance effects, a series of capacitors are used [1]. Following figure illustrates coaxial probe feeding.

Figure-2.6: Coaxial probe feed

(3) Aperture coupled feed

In this technique, two substrates are separated by a ground plane. A microstrip feed line is placed on the lower substrate and a slot made on the ground plane is used to couple the field lines and the radiating patch. The amount of coupling depends upon the shape, size and the position of the slot. A substrate with high dielectric constant is used at the bottom and low dielectric constant substrate is used at the top. This allows radiation from the patch to improve [1]. It is difficult to fabricate and has narrow bandwidth. This technique is shown in figure below.

Figure-2.7: Aperture coupled feed

(4) Proximity coupled feed

In proximity coupled feed method, a microstrip line is placed between two substrates, while the radiating patch is placed on the upper substrate. The feed line is open ended, thus resulting in a coupling which is capacitive in nature. A stub can be used to end the feed line; this helps in improving the bandwidth. This type of feeding results in higher bandwidth. The following figure demonstrates proximity coupled feed.

Figure-2.8: Proximity coupled feed

2.4 RFID Basics

RFID is a method of automatic identification which is gaining extensive attention due to its flexibility. It uses radio signals to identify objects. One of the most common methods for identification is to use RFID tags with unique identities, attached to the object that is to be detected. It is considered better than other tracking and detecting mechanisms, like bar codes, due to higher data processing speeds and longer distances [6].

2.4.1 History and Technology Background

RFID technology has been commercially available for over two decades, tracking its roots back to military identification Friend and Foe (IFF) systems of the 1940s [1]. Various theoretical explorations of RFID technology and some research papers were published in the early 1950s. In 1960s some researchers developed the first prototype systems. In 1970s, RFID was taken seriously and many researchers and developers took serious steps in developing the systems. During this period animal tracking using RFID was made commercially available [4]. In 1980s, besides animal tracking, RFID became available in Toll collection in highways. RFID system was further spread to rail applications and access control. It spread to various countries alongside America and became quite popular in Europe [4]. Currently, work is done in expansion of frequency spectrum, increasing the gain and read/write distance between transmitter and receiver.

ETC (Electronic Toll Collection) is an electronic automation toll collection system that was been a hot topic of research in recent years. RFID enabled labels are pasted on the vehicles. The center control computer can identify the road user by the information stored in the electronic label and deduct the toll from the road user advance stored card or bank account.

Figure-2.9: An example overview of the RFID ETC system

The RFID and ETC specifications have been issued, and the issue is being discussed all over the world. Several frequency bands are used in ETC, such as 900MHz, 2.4GHz and 5.8GHz as well. Pakistan has now joined the list of growing countries where RFID based electronic toll collection is in use. For now it has been introduced by NHA (National Highway Authority) in collaboration with NADRA (National Database and Registration Authority) on a few inter-city toll collection points and Peshawar - Islamabad M1 and Islamabad - Lahore M2 Motorways. The system which is installed in Pakistan, a vehicle will still have to stop at a booth but no human transaction between the vehicle occupants and toll booth operator is needed.

2.4.2 Basic components in an RFID system

An RFID system comprises of the following main components: (1) device (tag), (2) RFID enabled receiver, and (3) a database system.

(1) RFID Tag

RFID tags are also known as transponders. The word transponder is derived from transmitter and responder reveals the function of the device. The tag responds to the transmitted or communicated request for the data from the reader. The manner of communication between the reader and the tag is wireless, means across the space or air interface between the two components. An interrogator is often used as an alternative to the reader and interrogator is formed by combining reader with decoder and an interface. They are fabricated as low power integrated circuits suitable for interfacing to external coils or for utilizing "coil-on-chip" technology for data transfer and power generation [7].

(2) RFID enabled receiver

It is the device that is used to interrogate an RFID tag. The reader has an antenna that sends radio waves to the transmitter, and the transmitter replies back by sending back its data to the receiver. This received data is collected by the database system at the backend of the receiver antenna [6].

The reader can differ quite significantly in intricacy, depending on the types of tags being supported and the functions to be fulfilled. However, the overall function is to provide the means of communicating with the tags and facilitating data transfer. Functions perform by the reader may include quite complicated signal conditioning parity error checking and error correction. Once the signal from the transponder has been correctly received and decoded, algorithm maybe applied to decide whether the signal is a repeat transmission and may than instruct the transponder to end transmitting [1]. The overall architecture of this system is shown in figure below.

Figure-2.9: An overview of the RFID system architecture and components

2.4.3 Current uses

RFID is used in a variety of applications. Some current applications involving this technology are listed below:

Electronic toll collection

Railway car identification and tracking

Asset identification and tracking

Animal tagging

Toxic waste management

Drug industry

Logistics applications

Security

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